Very interesting!

From version 9979, you can split a way with the relation editor window open. There are 2 buttons in the upper left corner of the window. With the second from left you put the content of the relation window in JOSM This is what you have to do before splitting or connecting a road. With the most left button you synchronize JOSM and the relation window after splitting a road (P) or after connecting two roads (C) When you use these two buttons, the right way, you don’t get conflicts